Marriage therapist Lori Gottlieb claims that modern marriages are often fraught with incredibly high expectations, especially around sexual satisfaction. In other words, many married people churn with discontentment about how their spouse is not meeting their sexual needs. Today, Gottlieb says, we don't just want sex. It has to be sex that is "intimate," "transcendent" and "self-actualizing."

Even married couples who say they are satisfied with their sex lives often crave more. For example, Gottlieb mentions a study that asked participants who had had affairs why they did so. Fifty-six percent of the men and thirty-four percent of the women who admitted having affairs said they were "happy" or "very happy" in their partnerships, but cheated anyway.